Rather typical problem of our customers is running out of space in "tmpdir" running some large queries or ALTER TABLE queries. It is especially common with customers having "tmpdir" pointing to swapfs which is limited in side. So we shall do the following 1) Allow setting "tmpdir" without restarting the server. 2) Allow setting "tmpdir" on session basics. Note this task is linked to WL#1243 (Separate tmpdir for slave) which shall be trivial to do as soon as this task is done.